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WEB TUTORIAL 23.2
Primary Succession
Text Sections
Section 23.7 Succession in Communities, p. 408
Introduction
What is primary succession, and how does it occur? Recent data suggest that succession in Glacier Bay, Alaska, has followed at least three distinct pathways, each
of which will lead to a different climax community. This tutorial explores how succession varies among the Glacier Bay sites and shows long-term data on individual tree species at each site.
Learning Objectives
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Understand what primary succession is.
Know the stages of a successional pathway.
Narration
Primary Succession
What is primary succession, and how does it occur? Recent data suggest that succession in Glacier Bay, Alaska, has followed at least three distinct pathways, each
of which will lead to a different climax community. This tutorial explores how succession varies among the Glacier Bay sites and shows long-term data on individual tree species at each site.
In the far north end of the bay, the substrate has been exposed for less than 20
years. These communities do not have a continuous plant cover and are dominated
by willow and Dryas shrubs.
In the Uppermost Bay, the substrate has been exposed for 45 to 80 years. Alder and
cottonwood trees predominate in these areas.
In the Upper Bay, the substrate has been exposed for 100 years. These communities are a mixture of alder and spruce trees.
Communities in the Lower Bay have been undergoing succession for the longest
period; the substrate here has been exposed for 150 to 200 years. These communities consist of spruce and hemlock trees.
These observations inspired a hypothesis for the pattern of succession in Glacier
Bay. According to this hypothesis, these sites represent different stages along a single successional pathway. First, we see the arrival of photosynthesizing pioneer
species—in this case, bacteria and lichens—which can establish themselves in the
most barren environments. Photosynthesis is the foundation of almost all life on
Earth; as such, organisms that can perform photosynthesis must begin to flourish
in order for a community to develop.
Next we see an increase in “biomass” or the dry-weight of material produced by
living things. This is followed by a general movement toward longer-lived species.
The Dryas shrub can live for 50 years, but alders can live for 100 years, and spruce
trees can live for 700.
The growth of some later species is facilitated through the actions of earlier
species. The Dryas shrubs have bacteria in their roots that can fix nitrogen. This is
how a community of larger plants could get started in Glacier Bay, which lacked
nitrogen until the Dryas appeared.
Finally, some species are driven out by the actions of later species. The spruce trees
in the figure “shaded out” the smaller alder bushes, thus depriving them of the sunlight they needed for photosynthesis.
You should now be able to…
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State the difference between primary and secondary succession.
List the species that dominate at each site in Glacier Bay.
Describe the main stages in a successional pathway, using Glacier Bay as
an example.
Discuss the effects that existing species can have on subsequent species
during succession.